I read somewhere in the docs that the build happens in a cross compiler and not in the Pi itself, so it can build faster.
However, I am trying to build a nodejs app and it took the whole weekend and is still building. The build performance seems very similar to when I build directly on the RPI 2 model b.

We believe the build “hung”. The reason is not quite clear, but a few things have come up: bringing files with git clones which can hung due to unresponsiveness (this is the most likely one, this “hung” is accompanied by a “Still working…” message in the dashboard that will just not go away:D), another possibility is a qemu bug that results in builds being hung (we are looking into solution like fully arm build servers). The later is less likely to be your case because the build should automatically re-trigerr after 24h. If your case is the “git clone hung” can you please try removing all the git clone commands and copying the files some other way?
Whatever is the case please write back so I could provide support.

I will need a bit more specific information about what you are trying to build, in order to identify the issue. Could you describe it shortly? Alternatively if the project is not private could you point me to a repo?